Influences on Atmospheric Hydroxyl Radicals

The atmospheric lifetime of methane, currently ~12 years, is primarily due to oxidation by hydroxyl radicals. The concentration of hydroxyl radicals available to oxidize methane depends on the atmospheric conditions, including the concentrations of other atmospheric gases that compete with methane for hydroxyl radicals and gases which act as precursors to hydroxyl radicals. There is currently no framework for evaluating how the hydroxyl radical concentration, and therefore methane, will vary under future climate policies which address gases other than methane.

Some key atmospheric gases that compete with methane for hydroxyl radicals are hydrogen, non-methane volatile organic compounds (NMVOC), and carbon monoxide. The development of hydrogen as an energy source may result in fugitive hydrogen emissions, while carbon monoxide emissions may increase as wildfires become more prevalent. Conversely, nitrogen oxides (NO and NO₂) increase ozone (O₃) and therefore increase production of hydroxyl radicals. Estimates for the impacts that these gases have on methane are ~-1.3 Tg CH₄ per Tg NO₂ / year, ~0.15 Tg CH₄ per Tg C (in NMVOC form) / year, and ~0.1 Tg CH₄ per Tg CO / year (1). However, these estimates are highly uncertain, may be outdated with modern modeling updates, and may be spatially dependent.

The core problem to be addressed is the characterization of non-linear interdependencies between methane and the trace gases (NO_X, NMVOCs, CO, H₂) that influence the atmospheric oxidative capacity (driven by hydroxyl radicals). We require further understanding of the current relationships between these gases and predictions for what they are likely to be in future climate scenarios. 

Key goals include 1) synthesizing the future projections of select trace gases (using multiple models and scenarios), 2) comparing multiple models to analyze methane sensitivity to different gases and determine uncertainty estimates, and 3) assessing potential regional or local dependencies.

Success can be evaluated based on the derivation of multi-model estimates of methane sensitivity to select trace gases (NO₂, NMVOCs, CO, H₂) with rigorous uncertainty estimates.

If successful, improved understanding of atmospheric methane oxidation would:

• Lead to identification of climate policies that could avoid the lowering of atmospheric oxidation capacity (e.g., limiting H₂ leakage).
• Improve understanding of the relationships between key atmospheric trace gases, hydroxyl radical concentrations, and the methane budget.
• Rigorously quantify model sensitivities for different concentrations of trace gases.
• Rigorously quantify the impact on atmospheric methane for different future trajectories of trace gases.
• Shift understanding to recognize the role of trace gases in influencing the complexity and interdependencies of the climate system and associated policies. 
• Improve understanding about whether certain climate or air quality policies (such as reducing NO₂ or CO emissions) have significant co-benefits or risks such as reduced or increased methane.